This invention relates to a hammer drill bit for rock formations and is of the type that has a shaft which is connectable with a hammer drill or a drill rod and has a crown provided with hard metal studs each having a conical cutting tip. At least some of the studs are circumferential studs distributed along the crown circumference. These circumferential studs are all oriented, with their conical cutting tip, forwardly and away from the tool bit axis and their axis forms an acute angle with the tool bit axis.
Hammer drill bits of the above-outlined type are known in which the axes of all hard metal studs arranged on the crown circumference from an angle of 45.degree., 30.degree. or 37.degree. with the bit axis. In these known tool bits the axes of the other hard metal studs arranged on the crown are parallel to the tool bit axis and further, the conical cutting tip of the hard metal studs has an opening angle of 75.degree. to 90.degree.. In the known hammer drill bits, the hard metal studs are brazed or press-fitted into bores provided on the drill bit crown.
Hammer drill bits in which the axis of each hard metal stud arranged at the crown circumference forms an angle of 45.degree. with the tool bit axis wear rapidly, since the obliquely inserted hard metal studs break prematurely in view of the fact that the base of the borehole and one part of the lateral surface of the conical cutting tip of the hard metal studs are in a large measure parallel to one another, so that a substantial pressing force is exerted on these studs by the borehole base.
Also, the durability of the tool bits in which the axis of each hard metal stud arranged at the crown circumference forms an angle of 30.degree. with the tool bit axis is unsatisfactory, since the wall of the borehole and on part of the surface of the conical cutting tip of the hard metal studs inserted at an angle of 30.degree. extend essentially parallel to one another, so that the wall of the borehole exerts such a great pressure on the hard metal studs that the drill bit tends to jam in the borehole and the obliquely inserted hard metal studs are likely to break.
A certain improvement in the durability of the known hammer drill bits could be achieved by providing that the axis of each hard metal stud arranged at the crown circumference forms an angle of 37.degree. with the tool bit axis. Even in such an arrangement, however, the hard metal studs were found to break much too rapidly.
For avoiding a premature wear of the known hammer drill bit it has further been proposed to grind a clearance angle of approximately 3.degree. on the hard metal studs arranged at the crown circumference. Although, as a result, the hammer drill could operate without large pressure exerted thereon by the borehole wall, it is a disadvantage of this solution that with the above-noted grinding operation, a part of the crown material is necessarily removed. This enhances a breakage of the hard metal studs from their socket.